This invention relates to a multileaf collimator for use in a radiation system used to shape and control spatial distribution of the radiation field intensity.
Conventional radiation treatment of a tumor in a patient is carried out by planning the radiation beam angles and dosage, taking into consideration safety factors with respect to the patient's normal tissue and organs located in the path of the proposed radiation beam. The usual treatment field shapes result in a three-dimensional treatment volume which includes segments of normal tissue and organs (a safety margin around the tumor), thereby limiting the dose that can be given to the tumor. Cure rates for many tumors are a sensitive function of the dose they receive. The dose that can be delivered to the tumor can be increased if the portion of the normal tissue or organs receiving dose can be reduced. Techniques are under development to make the treatment volume conform more closely to the shape of the tumor volume. This permits higher dose to tumors and less damage to normal tissue and organs, with its attendant positive effects on the health of the patient. The techniques typically involve moving the jaw-blocks during treatment, scanning the radiation beam over the volume to be treated or using a multileaf collimator. Multileaf collimators can provide a similar function as the conventional jaw-blocks. In addition, each individual segment or leaf in a multileaf collimator is usually independently positionable. The radiation beam is directed at the ends and sides of the collimator leaves such that the beam is limited to the desired treatment area to be irradiated, while shielding the normal tissue and organs.
Radiation beam penumbra occurs in systems equipped with multileaf collimators at the edges of the radiation field where the radiation intensity decreases with distance from the full intensity region of field. This phenomenon is a combination of geometric penumbra due to the radiation source size and transmission penumbra due to penetration of the radiation beam through the ends of the multileaf collimator leaves. Geometric penumbra is a function of the source size, the thickness of the leaves, the distance of the leaves from the source and the distance of the reference plane from the source. Transmission penumbra is a function of material the leaves are made from, the thickness of the leaves and the energy of the radiation beam.
In the technical paper, "Design Principles of Telecobalt Collimators", W. H. Sutherland and C. W. Smith, Physics in Medicine and Biology, 22, 1189-1196 (1977), the authors clearly show that minimum geometric penumbra is produced when radiation collimators are pointed at the side of the radiation source.
The penumbra produced by square-end or simple curved-end linear-motion multileaf collimators at points equidistant from the central axis of the radiation beam is not equal. This can be explained as an effect of geometric penumbra. When the leaf is fully retracted from the central axis, the radiation field is defined by the portion of the leaf end furthest from the radiation source, the distal portion. In the fully extended position, the portion of the leaf end closest to the radiation source defines the radiation field, the proximal portion. The proximal portion of the extended leaf end produces greater geometric penumbra than the distal portion of the retracted leaf for positions equidistant from the central axis of the radiation field because the radiation source is perceived as larger from the proximal portion.
Typically, megavoltage radiation beams are very penetrating. Collimators and jaw-blocks that are used to sharply define the shape of radiation beam are typically made from high density, high atomic number materials and are usually several inches thick. If thinner sections, with less attenuation, are used then the edge of the radiation field is not defined as sharply, hence the transmission penumbra is larger.
U.S. Pat. No. 4,672,212 to Brahme discloses a multileaf collimator in which the entire leaf body is curved. The curved leaf follows a curved path of travel such that the flat leaf end is always tangent to the radius of an imaginary circle having its center at the radiation source. This configuration minimizes transmission penumbra. However, the curved leaf body results in complicated leaf mounting structures which are mechanically complex, physically large, difficult to retrofit onto existing systems and expensive to manufacture.
Linear motion multileaf collimators or jaw-blocks are easier to fabricate and assemble but their use typically produces larger penumbra. Leaf ends having simple curves of large radius produce acceptable penumbra for small field sizes. However, the transmission penumbra becomes progressively worse for larger fields. Leaf ends having small radii produce large penumbra for all field sizes. Also, penumbra for leaf ends at equidistant positions about the central axis are not equal.
The technical paper, "Analysis of the Field-Defining Properties of a Multileaf Collimator", N. Maleki and P. Kijewski, Medical Physics, 10, 518 (abstract) (1983), contains a figure which shows calculated penumbra for a range of simple, curved leaf-ends with a constant radius. A figure from the paper is reproduced here as FIG. 6. As discussed above, leaf ends having curves of large radius produce acceptable penumbra for small field sizes. However, the penumbra becomes progressively worse for larger fields. Leaf ends having curves of small radius produce large penumbra for all field sizes.
U.S. Pat. No. 4,868,843 to Nunan issued on Sep. 19, 1989, assigned to the assignee of the present invention, is hereby incorporated by reference thereto. Nunan discloses a multileaf collimator assembly which can be retrofitted as an accessory to existing systems. Alternatively, the Nunan multileaf collimator may be incorporated into the design of a new radiation system. The multiple leaves are independently positionable and travel in a straight line along the longitudinal axis of the individual leaves. This patent incorporates leaves with simple curved ends.
U.S. Pat. No. 4,534,052 to Milcamps describes a linear-motion jaw-block having a curved end. The jaw-block is movable only in a retractable direction with respect to the central axis of the radiation beam. The curved jaw-block end is defined by a simple arc of large radius having a center of curvature positioned on the proximal jaw-block surface, closest to the radiation source. The radiation beam of Milcamps is defined by a sharp edge at the intersection of the "active surface" and the distal surface when the jaw-block is at the furthest retracted position from the central axis of the radiation beam. This sharp edge readily transmits the penetrating radiation beam causing excessive transmission penumbra. Additionally, the asymmetric jaw-block "active surface" produces unacceptable penumbra if extended beyond the central axis of the radiation beam, by transmission of penetrating radiation through the sharp edge, at the intersection of the proximal surface, closest to the radiation source, and the "active surface".